The invention lies in the field of printing technology. More specifically, the invention pertains to ink supply control systems for web-fed and sheet-fed offset printing presses, and, in particular, to an ink key presetting system using a simulation of a printing unit with an ink film controller.
In offset printing, ink is supplied via an ink train, also referred to as the inker, from an ink fountain to a plate cylinder and then to a blanket cylinder, from which the ink is transferred to the print substrate (e.g. paper). The ink train includes an ink fountain roller, also referred to as an ink pickup roller, which picks up the ink at the ink fountain and transfers it to an ink ductor roller. The ductor roller oscillates between the ink fountain roller and an ink vibrator roller and thereby transfers the ink from the ink fountain roller to the vibrator roller. From there, the ink is transferred via distributor rollers to other vibrator rollers, which distribute the ink onto several ink form rollers. The ink form rollers ink the printing plate on the plate cylinder by depositing the ink onto the oleophilic surfaces on the plate. From there, the ink is transferred onto the rubber blanket in accordance with the image to be printed.
The amount of ink that is transferred is most important for the print quality. Too much ink in the ink train leads to smearing and blurring of the printed image. Too little ink leads to faint print and uneven distribution of ink color.
Different amounts of ink are required in various zones according to the image to be printed. In order to vary the ink feed laterally across the width of the inker, the ink supply leaving the ink fountain can be adjusted with ink keys. Each ink key thereby defines the ink supply for a respective zone. Depending on the type of printing unit, there may be provided any number of ink zones across the width of the inker. The number of ink zones and ink keys may vary from six to sixty, or even more.
The ink supply is subject to a vast number of variables. To begin with, due to the rheological characteristics of the ink, the relationship between the ink key feed gap and the amount of ink supplied at a given key is not linear. The type of printing ink, dampening agent, and paper, as well as process temperature and plant humidity influence the steady state behavior as well. Further, the various oscillating rollers in the ink train cause a substantial lateral distribution of the ink, so that the amount of ink supplied to a given zone at the rubber blanket is not only dependent on the ink key associated with that zone, but also on adjacent ink keys. In other words, as the ink travels from the ink fountain to the rubber blanket via several laterally oscillating vibrator rollers, a certain amount of ink bleeds from one zone to another. This phenomenon is referred to as the lateral coupling of the inker. Also, the final print application onto the substrate depends on the offset ratio, i.e., the ratio at which the ink is transferred from the rubber blanket onto the substrate (the ink film is split from the blanket, so that one part of the ink is transferred onto the paper and the other part is carried on by the blanket back into the inker). Typical ratios are 50:50, 30:70, 70:30, and so on.
The setpoint image coverage distribution is typically obtained from a plate scanner or a digital image setter file during makeready. However, due to the various system-dictated parameters, the actual setting for the ink key opening is substantially different from the setpoint ink film thickness. During the start of a printing job, therefore, a substantial amount of time passes before the inker is properly ready for the imprint. Also, when the plate cylinder and the rubber blanket are finally thrown on for imprint, a substantial number of signatures are passed through with improper ink coverage, thus leading to a substantial amount of waste.
It is accordingly an object of the invention to provide an ink key presetting system for offset printing machines, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows for more dependable and detailed makeready and enables ink-key presets that lead to quick and dependable preinking and printing machine runup with a minimum of waste.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of presetting an inker in an offset printing press, which comprises:
inputting an image coverage distribution for an image to be printed;
setting printed ink film commands in accordance with the image coverage distribution;
simulating an inking operation for calculating a simulated ink coverage distribution, and thereby driving a steady state error between the printed ink film commands and the ink coverage to zero and obtaining ink key presets; and
presetting the ink keys of the inker with the ink key presets at a start of an actual print job.
In accordance with an added feature of the invention, the ink coverage distribution is obtained by scanning with a plate scanner or from a digital (image setter) file.
In accordance with an additional feature of the invention, the inking operation is simulated as follows:
all inker paths in the inker are discretized with equal sized segments by dividing with lateral segments and circumferential segments;
the printing unit is advanced by one circumferential segment;
an amount of ink on each element is calculated after traversing a nip between two roller;
a lateral overlap of elements is calculated based on a lateral motion of a vibrator roll in a given inker path;
the advancing and calculating steps are repeated for a desired number of revolutions of the plate cylinder in the inker path.
In accordance with another feature of the invention, the amount of ink is calculated as a mass of ink by summing the ink films entering the nip from incoming roller paths (conservation of mass principle) and then dividing the summed mass between the two rollers defining the nip according to a preset split ratio.
In accordance with a concomitant feature of the invention, the sum of ink entering the nip is a weighted sum of overlapping elements. The weighting is proportional to an amount of overlap.
With the above and other objects in view there is also provided, in accordance with the invention, an ink control system for an inker of an offset printing press, the inker including an ink fountain for supplying ink to an ink train in proportion to respective settings of a plurality ink keys each aligned with a respective inker path along which ink is transferred from the ink fountain to a substrate to be printed, and the ink train having a plurality of mutually nipping rollers including one or more vibrator rollers, a plate cylinder, and a blanket cylinder for imprinting a given image onto the substrate, the ink control system comprising:
a device receiving input data representing an image coverage distribution for an image to be printed;
the device being programmed:
to simulate a proportional-integral controller connected to a plurality of actuators each adjusting a setting of a key opening of a respective ink key of an ink fountain;
to set printed ink film commands in accordance with the image coverage distribution;
to simulate an inking operation for calculating a simulated ink coverage distribution, feed back printing plant information for calculating a steady state error between the printed ink film commands and the ink coverage, and drive the steady state error to zero and obtain ink key presets; and
the device having an output outputting ink key presets of the inker representing the ink key presets at a start of an actual print job for printing the image.
In accordance with again another feature of the invention, there is provided a method of evaluating controller parameters for a given print job. The method comprises implementing the above-described method in a printing system with a closed loop color controller for evaluating controller parameters for a given print job, and optimizing the controller parameters of the printing system.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an ink key presetting system for an offset printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.